Blackberries, once established, are hard to eradicate unless you either dig up the crowns or spray them annually with a broad-spectrum herbicide or a product specifically for brush control. Killing the crowns, or nodes, takes diligence and persistence because any crown that is missed will produce blackberry canes in the spring.
Keep the blackberry canes, or vines, mowed down during the spring and summer months. This can be done with a lawnmower in small areas or you may need to use a brush hog for large or overgrown areas. Keep the area mowed during the entire spring and summer months.
Dig up all blackberry crowns you can locate. Each crown or node can produce a new plant. Dispose of the crowns in the trash or other area where they cannot begin to re-grow.
Mow the area and spray with a broad-spectrum herbicide or a product designated for brush control during the fall months. You can apply the product anytime in the fall before the first frost. Applying the herbicide to the canes during the spring and summer will not give the desired effect.